Priority is claimed with respect to Application No. 199 25 468.0 filed in Germany on Jun. 3, 1999, the disclosure of which is incorporated herein by reference.
The invention relates to a stray-field sensor for measuring dielectric properties of substances, having generating elements for generating an electrical field, and shielding elements for shielding the generated electrical field, the shielding elements being configured such that the electrical field is located at least partially outside of the shielding elements.
Stray-field sensors of this type are known. The known stray-field sensors employ an electromagnetic stray field to measure dielectric properties of substances located in the electromagnetic stray field. For example, these stray-field sensors can be used to measure the effective dielectric constant of a substance for indirectly drawing conclusions about the type of substance, its moisture content, density, etc. To promote a better understanding of these stray-field sensors, and particularly of the present invention, the general technical background of the sensors is discussed below.
The aforementioned stray-field sensors are more commonly referred to as dielectrometer sensors. Dielectrometer sensors of this type preferably generate an electrical, high-frequency AC field. The substance to be examined is then brought into the high-frequency AC field, which increases the displacement current. A capacitor made in this manner is preferably a component of a resonant circuit. This resonant rise of the field intensity increases the sensitivity of the dielectrometer sensor with respect to ascertaining changes in the dielectric constant. A dielectric constant that increases when the substance to be examined is brought in lowers the resonant frequency, while losses in the substance to be examined damp the resonance.
There are two possible ways to examine the substance. First, the substance to be examined can be brought into the resonator. This method is known from, for example, from German application DE 197 05 260 A1 and the associated supplemental application DE 197 34 978 A1, both owned by the assignee of the present application. In the apparatuses described therein, the resonator of the dielectrometer sensor is disposed in a housing having an entrance opening and an exit opening. A rope of tobacco, particularly a cigarette rope, can be guided through the entrance or exit opening of the apparatus described here. Thus, in this apparatus, when the cigarette rope is guided through the resonator, this influences the resonant frequency of the resonant circuit, and permits a measurement of the properties of the cigarette rope, such as its moisture content or density. The cited documents also describe a method (not reiterated here) for precisely measuring these resonance shifts.
The cited arrangements and methods for measuring the density of a substance, particularly for measuring the density of a tobacco rope, have been proven effective because the cited methods and arrangements can be used to measure density in a control circuit, which simultaneously influences the density. Hence, these apparatuses can be used in the precise determination of the density of a substance to be examined, for example, the aforementioned tobacco rope.
The method described in the cited documents measures the resonance value at at least two points on the resonance curve of the resonator through which the substance is guided. In addition to the average value of the measured values and the difference, the dielectric properties, such as the moisture content of the substance and, consequently, the density of the substance, can be calculated.
In the tobacco-processing industry, it is particularly desirable to measure the moisture content of the substance to be examined as early as possible. Thus, in the tobacco-processing industry, it is desirable to examine the tobacco while it is still in bulk form, or before it has been rolled. This is because it is only at this point that the moisture content of the tobacco can still be influenced satisfactorily. If the tobacco has already been processed into a rope, the moisture content of the processed tobacco can no longer be influenced satisfactorily. It is therefore necessary to guide the substance to be examined past the resonator, which emits a stray field, rather than insert the substance as a sample into the resonator itself. The present invention, notably the stray-field sensor of the present invention, relates to this type of embodiment of a resonator.
In stray-field sensors of this type, the substance to be examined is guided through a stray field present outside of the actual resonator. For this purpose, the resonator is preferably embodied such that its electrical field is at least partially located outside of a shielding element, while the remainder of the electrical field, and the magnetic field, are located within the shielding element, so they do not interact with the substance to be examined, for example, the bulk tobacco to be examined. Thus, in the stray-field sensors of the type mentioned at the outset, only a portion of the electrical field, which is located inside the shielding element, is coupled into the outside space. Measuring in the microwave range has proven especially advantageous in measuring the moisture content of substances to be examined, for example the moisture content of tobacco.
For utilizing such stray-field sensors in precisely ascertaining the aforementioned losses in the substance to be examined, it is necessary to avoid emission losses of the stray-field sensor. This is imperative because, in non-magnetic materials, besides the losses in the substance to be examined, only dielectric losses occur in addition to known ohmic losses in the conductors. Dielectric losses are the objective of the measurement. It is therefore advisable to suppress the emission losses to the greatest possible extent. A suppression of the emission losses means that the far field of the electrical field is suppressed, so only a near field is still present in the vicinity of the stray-field sensor. The aforementioned emission losses are notably avoided if the emission components of the far field are canceled out for as many angular positions as possible due to their phase opposition. This mutual cancellation of the emission components in the far field is only possible, however, if the diameter of the opening through which the stray field exits the shielding elements of the stray-field sensor is significantly smaller than the used wavelength. A drawback of this is that, because of the small diameter of the stray-field openings in the stray-field sensor, measurements of the moisture content of the substance to be examined, for example, can only be taken in this region. Therefore, stray-field sensors of this design, that is, stray-field sensors having small openings, produce only point-wise measurements of the dielectric constants of the substance to be examined. This is also disadvantageous if the substance to be examined is non-homogeneous. This lack of homogeneity is characteristic of bulk goods, for example. In loose tobacco, for example, as is used by the tobacco-processing industry, the arbitrary arrangement of the individual tobacco fibers in the loose tobacco presents a severe non-homogeneity. Stray-field sensors that measure point-wise can only provide imprecise measurements of the moisture content of such non-homogeneous tobacco.
To overcome the above-described disadvantage, it has been proposed to use a plurality of such stray-field sensors. The use of a plurality of stray-field sensors or resonators permits the approximate determination of the moisture content of leaf tobacco, for example, which is often partly not in climatic equilibrium, even with a non-uniform moisture distribution in the individual tobacco leaf; specifically, the plurality of stray-field sensors determines the moisture content. An arrangement of numerous sensors is known from, for example, EP-A-0 558 759 A1.
A drawback of the described arrangement of a plurality of resonators or stray-field sensors is that, if measurements are taken in the microwave range, for example, the microwaves must be divided via a power divider and re-mixed behind the resonators or sensors. This requires a high outlay for components. Furthermore, this type of measuring arrangement requires the use of high-quality resonators to ensure that all resonators operate at the same frequency. All in all, overcoming the problems of emissions in this arrangement is associated with a high technical outlay.
It is therefore an object of the invention to improve a stray-field sensor of the type mentioned at the outset to avoid the above-described disadvantages and make available a stray-field sensor that can reliably examine the dielectric properties of even non-homogeneous substances or bulk items, particularly their moisture content, and has a simple design.
In a stray-field sensor of the type mentioned at the outset, this object is accomplished in that the shielding elements have at least two openings for coupling out the electrical field.
The coupling out of the stray field at two locations of the shielding elements advantageously permits the examination of the dielectric properties of non-homogeneous substances, even with small openings/locations through which the stray field exits to the outside, namely comes into contact with the substance to be examined. These dielectric properties then reveal the moisture content of the substance to be examined. Thus, with the invention, it is possible to use a sufficient number of locations for coupling out the electrical field or coupling the electrical field with the substance to be examined in order to take the mean of the individual measured values, and thereby obtain a more precise measured value than was possible in the aforementioned known stray field sensors. This is even the case with non-homogeneous substances, such as leaf tobacco, which has a non-homogeneous moisture and density distribution. In particular, the invention also avoids the disadvantages ensuing from the arrangement of a plurality of independent stray-field sensors according to EP-A-0 558 759 A1, because the invention eliminates the necessity of an identical design of the individual sensors and the division and re-mixing of the electromagnetic waves in front of and behind the sensor, respectively.
The invention employs a single stray-field sensor, or a single stray-field resonator, and simultaneously couples the stray field out into the outside space at a number of locations. Thus, it is possible to cover a large surface area with a resonator of the invention without the drawbacks of large openings, i.e., emissions. The invention is therefore well-suited for use in the tobacco-processing industry, in which, as mentioned several times already, it is critical to analyze the loose tobacco or leaf tobacco, which is non-homogeneous with respect to density and moisture content. With the invention, an analysis of this type is possible in a simple manner, and with high precision.
The electromagnetic field generated by the generating elements is preferably a high-frequency AC field. The generating elements are preferably embodied such that the stray-field sensor constitutes a resonator, that is, the high-frequency AC field is in resonance. The measuring method used in this connection, and the precise taking of measurements, are known from the cited German patent applications DE 197 05 260 A1 and DE 197 34 978 A1. They are not described again here, because a person of skill in the field of microwave measuring technology can easily apply the method described therein to the present invention.
In a preferred embodiment of the invention, the shielding elements are embodied as a housing for the generating elements. The openings are configured as holes in the housing. The housing preferably comprises metal.
In an especially preferred embodiment, the electrical field is coupled out at points of identical amplitude of the electrical field due to the arrangement of the openings in the shielding elements or the housing at the points of identical amplitude. It is especially preferable for the locations of identical amplitude to be identical-polarity maxima of the electrical field. Due to the interruption of the shielding of the resonant electrical field at locations of the half-wave electrical-field maxima of the same polarity, a portion of the electrical field is coupled as a stray field outside of the shielding elements such that each coupling out, in and of itself, does not radiate, that is, no far field is present. In this way, the present embodiment of the invention advantageously limits the stray field to the immediate vicinity of the stray-field sensor with this embodiment. The identical poling of the electrical stray-field components exiting the stray-field sensor prevents a long connecting field line between the stray-field components exiting the different openings in the shielding elements because there is no difference in potential between stray-field components exiting the individual openings in the shielding elements; such a field line would otherwise result in an emission.
A particular advantage of this embodiment is that there is no need for rows of numerous stray-field sensors or stray-field resonators. Instead, with the invention, a single stray-field resonator is sufficient in this embodiment to make available a plurality of measuring points by way of the numerous openings, so non-homogeneous objects can also be measured. Thus, this embodiment also permits the measurement of, for example, leaf tobacco or loose tobacco having a non-homogeneous density and moisture distribution.
In a further embodiment of the invention, the generating elements for generating the resonant electrical field are embodied as a waveguide. The waveguide is preferably embodied as a line resonator through a short-circuit at its two ends. The length of the waveguide is preferably selected to be a whole-number multiple of one-half of the wavelength. The waveguide length is especially preferably three times one-half of the wavelength, or, even more preferably, at least ten times one-half of the wavelength at resonance. In these embodiments, simple means can assure an arbitrary number of measurement points. To provide an entire surface with measurement points, for example, the line resonator can have a meandering design. This permits a maximum number of measurement points to be provided per surface. The surface is preferably the surface of a wall past which the bulk item, for example, the loose tobacco to be measured, is guided.
A different preferred embodiment includes a cross-shaped waveguide. Here, it is especially preferred for the whole-number multiple of one-half of the wavelength, which constitutes the total length of the cross-shaped waveguide, to be an uneven whole-number multiple of one-half of the wavelength. This is because, in this type of embodiment, the electrical field, for example a microwave field, can be coupled into the center of the line. This type of arrangement, which can also be selected in a meandering arrangement of the waveguide, prevents additional resonances at different frequencies from the desired resonant frequency if the waveguide is fairly long. Such resonances can cause mutually-interfering resonances if the waveguide is fairly long, or if there are many openings in the shielding elements, for example, in the housing of the resonator acting as a generating element. In this regard, selecting the waveguide length to be an uneven multiple of one-half of the wavelength is an advantageous countermeasure. A commensurate field maximum exists in the center of the line resonator. If the field is coupled into the line resonator at this point, all resonances for which n is even are suppressed, because they have a zero field value. The suppression of undesired resonances is also assured in other embodiments that provide an arbitrary coupling of the electrical field, which is symmetrical to the center of the waveguide, into the line resonator.
In the aforementioned cross-shaped arrangement of the line resonator, it is especially preferable for the length of each leg of the cross to correspond to five times one-half of the wavelength of the resonance. This enables the provision of five measurement points, or openings, in the housing for the line resonator, including a measurement point in the center. This embodiment of the invention is particularly simple and compact, and simultaneously provides a sufficient number of measurement points.
In the aforementioned cross-shaped embodiment of the waveguide, the spacing between the actual line resonator and the housing surrounding the line resonator is preferably established by pins that are connected to the line resonator and extend perpendicular thereto, the pins being respectively disposed at the points where the electrical field is coupled out into the outside space. This allows the stray field to be coupled out especially simply into the outside space in order to interact with the substance to be measured. It must be considered that the line length of the pins leading from the line resonator to the respective measuring window, or opening contributes to the length of the waveguide or line resonator.
It is particularly preferable for the waveguide to be highly resistive. This attains an especially high quality of the line resonator formed by the waveguide, so the measurement results are likewise of high quality.
In a further embodiment of the invention, the electrical field is coupled into the housing through openings in the housing surrounding the line resonator. Corresponding antennas serve in the coupling-in and coupling-out processes. As mentioned above, the electrical field is preferably coupled into the center of the resonator. The coupling out can take place in the vicinity of the line ends of the line resonator, although this is not absolutely necessary, because the conversion into the suppressed interference modes is low there. On a related note, a symmetrical arrangement of the coupling-out antennas, i.e. a symmetrical coupling out, avoids such a conversion.
In a preferred embodiment, a cavity resonator constitutes the generating element for generating the electrical resonant field. In this case as well, the electrical field is preferably coupled into the center of the cavity resonator. In the cavity resonator, the generated electrical resonant field is coupled into the outside space, or the object to be measured, via antennas. These antennas are preferably embodied as cylinders comprising a dielectric material, but more preferably as Plexiglas cylinders.